Frequency shift transmitter

ABSTRACT

A frequency shift keying oscillator comprising filter circuits and a positive feedback circuit. First resistors and reactance elements in each filter determine the natural frequency of the oscillator. Keying signals selectively switch second resistors in parallel with the first resistors to change the natural frequency. The percentage resistance variations produced by the keying signals are substantially equal. This equalizes the steady-state gains of the oscillator in its two modes so that no amplitude discontinuities occur in the output signal as the oscillator switches from one mode to the other. As the reactor in each filter is a constant value element, no phase discontinuities occur. Hence, the output signal is a continuous, fundamental sinusoidal signal which has two mutually exclusive frequencies.

FILTER 20 Umted States Patent [151 3,641,460 Holsinger 1 Feb. 8, 1972[54] FREQUENCY SHIFT TRANSMITTER 3,482,l88 12/1969 Crouse...., ..33l/l35[72] Inventor: Jerry L. Holsinger, Lexington, Mass. Primary EmWMr' JohnKominski [73] Assignee: lntertel, lnc., Burlington, Mass. An myCesari dMCKenna I [22 Filed: Nov. 9, 1970 [57] ABSTRACT Appl- 87,706 A frequencyshift keying oscillator comprising filter circuits 7 and a positivefeedback circuit. First resistors and reactance 52 US. c1. ..331/110,331/141 elements in each film determine the "3mm! f'equency [51] Int CL03', 5/26 oscillator. Keying signals selectively switch second resistorsin 581 Fieldoisearch ..331/11o 135-141 Pmne' with chanBe the "3mm!330/107. 333/0 T 70 cy. The percentage resistance variations produced bythe keying signals are substantially equal. This equalizes the steady-56 R "mus state gains of the oscillator in its two modes so that no amld CM plimde discontinuities occur in the output signal as the oscillarrSTATES PATENTS tor switches from one mode to the other. As the reactorin each filter is a constant value element, no phase discontinui-2,820,903 1/1958 Roulston etal ..33l/l4l ties occun Hence, the outputSig-3| is a continuous. funda 2,983,380 5,196] mental sinusoidal signalwhich has two mutually exclusive 3,257,611 6/1966 McKun..... ....331/141fmquenciw 3,378,785 4/1968 Nordahl.... ....330/l07 1 3,396,346 8/1968Richman ..33 1/1 35 4 Claims, 3 Drawing Figures KEY|NG 0 CIRCUIT 2 1 BZHIGH-PASS LOW-PASS 1 l FILTER 22 PATENTEUFEB 81972 she-A50 SHEET 1 OF 2KEYING CIRCUIT //0 v E Q-E11 5 Z I HIGH-PASS LOW-PASS l FILTER 20 FILTER22 UTILIZATSON DEVICE LOW PASS l-HGH PASS FILTEAR 70 FILTEB a0 Afnvenfor F I U- 2 Jerry L, A D/Singe A tor-neg;

This invention generally relates to oscillators,-more specifically tooscillators using filter circuits for frequency shift keyingapplications.

There are several systems and methods for transferring data between tworemote locations. With frequency shift keying systems, a modulatorswitches a carrier between two or more discrete frequencies in responseto data signals in binaryform. Any two frequencies can be used for thecarrier. Closely related frequencies in the audio spectrum'are generallyused when the telephone system serves as the transmission medium. Also,a frequency-shift subcarrier in theaudio band mayibe used to modulate aradio frequency'carrier. The present invention is particularly directedto systems using such lowfrequency carriers or subcarriers.

When a data source feeds data to a frequency-shift keying transmitter,control and oscillator circuits 'in the transmitter produce the propersequence of frequency-shifted signals. The timing of shifts between thetwo frequencies depends on the timing of the data signals; thus, it isgenerally unrelatedto the phase of the carrier.

The audio signal sequences can be produced by two more oscillatorcircuits in conjunction with a-circuitfor. generating keying signals foreach characternln one arrangement, two oscillators independently andcontinuously generate the two audio signals. The keying circuitselectively and alternately switches the output from each oscillatorinto atransmitter. In another arrangement, a single oscillatorenergizes'frequency multipliers, dividers, or bothto'producethe'twofrequencies. The keying circuit again couples oneor'the other audio signal to the transmitter. in still anotherarrangement, the keying circuit turns two oscillators on and off in amutually exclusive fashion. Both oscillators are directly connected'tothe transmitter. Other systems use a keying circuit which controls andchanges frequency determining parametersto shiftanoscillator outputbetween two frequencies.

In these prior systems, phase and amplitudediscontinuities exist as thekeying circuit shifts the oscillato'ror related circuits from onefrequency to another. These discontinuities produce transients whichappear in the transmitter output. Transients can be tolerated in systemswhere relatively slow transfer rates exist (i.e each keying circuitchange occurs after several cycles of the carrier). in datapro'cessingand other systems, however, data transfer rates of 1,800 baudar'edesirable. At these rates, each keying circuit change can occur after afraction of a cycle. Under these conditions, the transients can exceedthe time the keying circuit produces one or the other of the two keyingsignals. As a result, prior systems incorporate rather complex signalconditioning circuits for reducing the transients in the output signalto acceptable levels to assure that .the systems transfer the dataaccurately.

In certain prior systems, the oscillator circuit shifts between two veryhigh frequencies. For example, an oscillator circuit might shift betweena 20.0 kHz. frequency and a 21.0 kHz. frequency. After the resultingswitching transients are filtered, the high frequency signal isconverted to an audio signal. in another approach, a high-frequencycrystal oscillator generating for example, a 1 ml-lz. center frequencysignal shifts between two frequencies. A digital dividing networkconverts these high-frequency signals to audiofrequency square waveoutputs which must be converted to a sinusoidal output signal. Bothsystems rely on the higher oscillator frequency to pennit increased datatransfer rates. However, they require expensive and complex filteringand frequency conversion circuits to obtain a fundamental sinusoidalsignal at the desired transmission frequencies.

In another system, one of two sources of different voltages energizesthe capacitors in the resistive-capacitor timing circuit for. afree-running multivibrator in response to keying signals. When thehigher voltage energizes the multivibrator, it

cy-shift'keying'oscillator of the resistance-rai feedback circuit. Theoscillation frequency 'de'te y not change the loop gain. Ace""operatesat ahigher frequencyfThis system idoes reduce switchingtransients'because' nophase 'shifts occur. However. the multivibratorproduces a square wavsi'g n'al, so signif cant filtering circuits arenecessary to"obtain"sinusoiclal output signals. 7

Therefore, itis an object of this invention 'to 'provide' a"frequencyshift keying oscillator which piovid'essinusoidal outputsignals. v Another object of this invention is to 'provid ea'frequencyshift keying oscillator which is ca pable of providing sinusoidal outputsignals without filtering.

It is another object of this inve'ntion to provide a simplifiedfrequency shift keying oscillator which is capable of transmitting dataat increased rates.

*SUMMARY 1 have found that transients cari'be eliminated i-a fh'e'iiu H3 Yb?- The oscillator includes high and low-pass'j 'filtersfand apositive the resistive "and reactive elements'in the 'filtersfiFrequency"shifts are obtained by changing the values resistances in the'filt'ers. Changes in the resistances do 'not directlyjcaiise'instan- 25"t'aheous changes in any of the voltages arduin'd the oscillator loopand,'if the changes are correctly prapbmo'ne'd; they do of transients.

This" invention "is'pointedout "with pai't' "tliai'it i the u pendedcIaimsJA more thorough iinderst'a ding arabqve'ahd BRIEF DESCRIPTION OFTHE DRAWINGS FIG. 1 is aschemat'idof'one embodiment 'of a freque'ncyshift keying circuifincorporating thisinveiition; I

FIG. 2'is a schematic of anmheroscinam 'ciicuit adapted for use inthecircuit'shown in FIG. 1; and v 7 FIG. 3 is 'a schematic of yet anotheroscillator circuit adapted for use in the circuits'hown 'in'FlG. 1.

DESCRIPTION OF lL'LU'STRATlVEEMBODIMENTS As shown in FIG. 1, afrequency-shift keyii'igdata'transmission system comprises a keyingcircuit 10, anoscillato'nn'and 'a utilization device 14. The keyingcircuit ro gdflamtigtr sequences which alternate betweeh f'two 'distinctlevels in response to digital infonnation, coiiit'rolsignals for radete'isgraphy or other data signals. When'the key'irig circu' energi'zesan input terminal 16 with these control or key signals, the frequency ofthe output signal at a terminal l'8 shifts between two values inaccordance with the'ke'yir'igs'ignals.

The oscillator 12 comprises two filters 20 and 22 c ected to aninverting input terminal 24 for an operational amplifier 26. Theamplifier 2 6 energizes the utilization device 14 while the filter 22,which comprises a capacitor 28 and a resistance unit R couples a portionof the output signal to the inverting input 24. This negative feedbacksignal varies with signal frequency in such a manner that the filter 22is an active, lowpass filter.

. The resistance unit R comprises a resistor 30 and a parallel circuitcomprising a resistor 32 and a field effect transistor (FET) 34 inseries. Signals from the keying unit 10 energize the F ET 34 through aresistor 36 so the value of the resistance element R changes in responseto keying signals. I

The other filter 20 comprises a capacitor 38 and resistance unit Rincluding aresistor 40 and a parallel circuit including a resistor 42and FET 44 in series. Aresistor 46 couples the keying unit 10 and theFET 44. Therefore, the resistance unit 22 comprises the resistors 40 and42in parallel when the FET 44 conducts and only the resistor 40 when theFET 44 does not conduct. This filter 20 is a high-pass filter.

A portion of the output signals from the amplifier26 are positively fedback to the capacitor 38. The feedback signals are produced by seriesresistors 48 and 50 and an amplifier 52. A limiting circuit 54 controlsthe magnitude of the feedback signal while the amplifier 52 inverts thesignal to provide the positive feedback to the input of the high-passfilter 20. A resistor 56 provides negative feedback for the amplifier52.

The limiting circuit 54 comprises a voltage divider including resistors58, 60, 62 and 64 in series between a positive and negative potentialsource, and diodes 66 and 68. The junction of the resistors 60 and 62 isgrounded. The diodes 66 and 68 are connected to the junctions of theresistors 58 and 60 and of the resistors 62 and 64, respectively. Bothdiodes are connected to the junction of the resistors 48 and 50 andpoled to conduct when the voltage at the terminal 34 exceeds a positiveor negative maximum. For example, if the feedback voltage increasespositively and tends to exceed the voltage at the junction of theresistors 58 and 60 plus the forward breakdown voltage of the diode 66,the diode 66 conducts and limits further increases.

Feedback circuit parameters are determined so the amplifiers 26 and 52operate linearly. in addition, the individual amplifiers 26 and 52 areoperated so the positive feedback assures oscillation.

Using a standard analysis for a transfer function, (see Mitra, Analysisand Synthesis of Linear Active Networks, John Wiley & Sons, Inc., NewYork, I969) the transfer function for the oscillator unit 12 can beshown to have the following form where R and R represent fixedresistance values:

l( /Q where k is a proportionality constant and is greater than l/Q, wis the natural frequency for the circuit, and

Equation (2) shows that the values of the resistance elements R and Rand the capacitors 28 and 38 determine the natural frequency. Equation(3) shows that the Q of the circuit can be modified by varying otherparameters in the circuit independently of the natural frequency. l havefound that the resistance elements R and R can be modified withincertain constraints so that the output signal frequency shifts withoutany phase displacement or discontinuities. First, the capacitances ofthe capacitors 28 and 36 must remain constant. Secondly, thesteady-state voltages across the capacitor 28 and the capacitor 38 mustnot change after the output signal frequency shifts.

The circuit shown in FIG. 1 satisfies both constraints. Only theresistance units R and R change values. The capacitors 28 and 38 haveconstant values. Further, the overall gain in the circuit assures thatthe steady-state voltages across each capacitor do not changeappreciably by switching the resistance units R and R when theresistance values are changed by approximately the same percentage orper unit value; i.e., AR IR AR /R where AR, and AR represent theresistance changes.

When the capacitors remain at the same value, the stored energy isconstant during the switching time. As a result, no phase shifts occur.If the steady-state gains remain the same, both frequency signals havethe same magnitude. Therefore, the output at the terminal 18 is acontinuous first-order function when the keying circuit changes thefrequencies; no transients occur. Further, the oscillator 12 produces afundamental sinusoidal signal, so no additional filter circuits arenecessary for eliminating harmonics.

As a specific example, the oscillator 10 shown in FIG. 1 has beenconstructed with the following circuit parameters:

High-Puss Filter 10 Low-Pass Filter 22 Capacitor 28=0.0l gl'ds ResistorJ0=7.lt$ kll Resistor 3Z=79.5 k1) Feedback Circuit 28 Resistor 48=l0 ktlResistor $6=2l Ul Voltage at junction of resistors 58 and 60.62und64=+3.3

This specific oscillator alternately energizes the utiligation device 14with a 2,025 Hz. and a 2,225 Hz. signal as the keying circuit 10disables and enables the F ETs 40 and 50.

Normally, highand low-pass filters comprise resistors and capacitors butthey can also comprise resistors and inductors. FIG. 2 illustrates oneembodiment of such a frequency-shift keying oscillator which usesinductors. Specifically, a low-pass filter 70 comprises an inductor 72in series with a resistor 74. A resistor 76 and FET 78 are in parallelwith the resistor 74. The FET 78 is resistively coupled to the terminal16.

A high-pass active filter 80 constitutes a negative feedback loop aroundan operational amplifier 82 and comprises an inductor 84 and a resistor86. Another resistor 88 is selectively switched in parallel with theresistor 86 by an FET also coupled to the terminal 16. A feedbackcircuit 92, analogous to the feedback circuit in FIG. 1, connects theoutput of the amplifier 82 to the inductor 72 in the low-pass filter 70.

Again, it can be shown that the transfer function for this circuit hasthe form:

and the values of R and R represent the combined values of the resistors74 and 76 and of the resistors 86 and 88 when the FETs 78 and 90conduct.

Analogous criteria must be met to obtain a continuous output signal.That is, the stored energy in each inductor must remain constant duringthe switching interval, and the steadystate current value through eachinductor must be the same for either frequency. When both conditions aremet, the oscillator produces a continuous output signal. Switching doesnot introduce any phase shift or amplitude variations so a fundamentalsinusoidal output signal is obtained without filtering.

FIG. 1 illustrates an oscillator with two operational amplifiers. Apreferred oscillator using RC filters and a single operational amplifieris shown in F IG. 3. A high-pass filter comprises a capacitor 102 inseries with the resistor 104. A resistor 106 and an FET 108 are inparallel with the resistor 104. The high-pass filter 100 energizes alow-pass filter 110 comprising a capacitor 112 and resistor 114 inparallel and connected between the output from the filter and ground.Another resistor 1 16 is coupled to ground by a FET 118. Both FETs 108and 118 are resistively connected to the terminal 16.

The high-pass filter 100 also energizes the noninverting input of anoperational amplifier 120. A voltage divider comprising a resistor 122and a resistor 124 in series between the output terminal of theamplifier and ground controls the amplifier gain. This signal voltageacross the resistor 124 energizes the inverting input of the amplifier120 and maintains a zero difference voltage across the amplifier inputs.A feed- Only the resistor values are changed by switching so thecapacitance values are constant during the switching interval. Further,the per unit resistance variations are the same for both filters.

Low-Pass Filter "0 Capacitor ll2=0.0l pfd. Resistor ll4=l5.l l5.lResistor ll6=80.6 kfl High-Pass Filter I00 Capacitor 102:0.0033 ufd.Resistor [04:45.3 kn Resistor l06=237 kn Amplifier Circuit X ModelResistor l22=6.34 kn Resistor 12%| .0 k9 Feedback resistor 126=L0 k0 709Operation Amplifier When the circuit is energized, it alternatelygenerates a 1,070 Hz. and a 1270 Hz. signal as the conduction of theFETs 100 and 108 alternately changes in response to keying signals.

ln addition to being simpler and less expensive to construct, theoscillator shown in FIG. 3 should be more stable. All the frequencydetermining elements are external to the amplifier 120, so apparentlyamplifier variations caused by environmental and circuit variations havea reduced effect. Commercially available capacitors and resistors havestable thermal and electrical properties. As a result, the requirementsfor a satisfactory operational amplifier can be less stringent. Thisallows still further economies in manufacture.

in summary, highand low-pass filters can be combined in an oscillatorcircuit. In accordance with my invention, l have found that thefrequency of oscillation can be changed by modifying the resistancevalues of each filter simultaneously with several advantages. Primarily,the oscillator frequency shifts to a new value without any significantphase or amplitude discontinuities. Therefore, transients aresubstantially eliminated. Further, the oscillator generates afundamental sinusoidal signal and obviates the need for complex andexpensive filtering used in the prior oscillators.

These three specific embodiments are illustrative only. Others can beconstructed and arranged to obtain a frequency shift keying oscillatorwith the same results. For example, the output signal is fed back to alow-pass filter in the circuit of FIG. 2 and to high-pass filters in thecircuit of FIGS. 1 and 3. Therefore, the location and orientation of thevarious filters depends upon particular filter configurations which areincluded in the oscillator unit. The parameters may be selected so theoscillator generates a signal with frequencies outside the audiospectrum even though these specific examples describe oscillators forgenerating audio signals. Further, the resistance values may be changedby other means than the specifically described parallel resistors andFETs.

Therefore, it is the object of the appended claims to cover all suchmodifications and variations as come within the true spirit and scope ofthe invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An oscillator for mutually exclusively generating two frequencyoutput signals in response to control signals, said oscillatorcomprising:

A. a first operational amplifier,

B. an active low-pass filter comprising a first resistance means andfirst capacitor in parallel connected between the output and invertinginput of said amplifier, said first resistance means having first andsecond values,

C. a high-pass filter comprising a second capacitor and a secondresistance means in series with the inverting input of said firstamplifier,

D. a feedback circuit including a second operational amplifier forinverting signals from the output of said first amplifier and applyingthe feedback signals to said high-pass filter, said feedback circuitadditionally including means for limiting the feedback signal to a valuewithin the linear operating regions of said operational amplifiers, and

E. means for switching said first and second resistance means betweenthe first and second values simultaneously in response to controlsignals to thereby vary the oscillator output frequency withoutproducing significant transients.

2. An oscillator circuit as recited in claim 1 wherein said first andsecond resistance means each comprise a first resistor and, in paralleltherewith, switching means and a second resistor in series, said firstand second resistor being paralleled in response to first controlsignals to thereby increase the natural frequency of said oscillator.

3. An oscillator for mutually exclusively generating first and secondfrequency signals in response to keying signals, said oscillatorcomprising:

A. a first operational amplifier,

B. a low-pass filter including a first inductor and a first resistancemeans in series with said first operational amplifer, said firstresistance means having first and second values,

C. a high-pass filter including a second inductor and a secondresistance means in parallel and connected between the output andinverting input of said first amplifiers, said second resistance meanshaving first and second values,

D. a feedback circuit including a second operational amplifier forinverting signals from the output of said first amplifier and applyingthe feedback signals to said low-pass filter, said feedback circuitadditionally including means for limiting the feedback signal to a valuewithin the linear operating regions of said operational amplifiers, and

E. means responsive to the control signals for simultaneously switchingsaid first and second resistance means between their first and secondvalues simultaneously in response to control signals to thereby vary theoscillator output frequency.

4. An oscillator as recited in claim 3 wherein each of said first andsecond resistance means comprises a first resistor and, in paralleltherewith, switching means and a second resistor in series, said firstand second resistors being paralleled in response to first controlsignals to thereby increase the natural frequency of said oscillator.

* i i i

1. An oscillator for mutually exclusively generating two frequency output signals in response to control signals, said oscillator comprising: A. a first operational amplifier, B. an active low-pass filter comprising a first resistance means and first capacitor in parallel connected between the output and inverting input of said amplifier, said first resistance means having first and second values, C. a high-pass filter comprising a second capacitor and a second resistance means in series with the inverting input of said first amplifier, D. a feedback circuit including a second operational amplifier for inverting signals from the output of said first amplifier and applying the feedback signals to said high-pass filter, said feedback circuit additionally including means for limiting the feedback signal to a value within the linear operating regions of said operational amplifiers, and E. means for switching said first and second resistance means between the first and second values simultaneously in response to control signals to thereby vary the oscillator output frequency without producing significant transients.
 2. An oscillator circuit as recited in claim 1 wherein said first and second resistance means each comprise a first resistor and, in parallel therewith, switching means and a second resistor in series, said first and second resistor being paralleled in response to first control signals to thereby increase the natural frequency of said oscillator.
 3. An oscillator for mutually exclusively generating first and second frequency signals in response to keying signals, said oscillator comprising: A. a first operational amplifier, B. a low-pass filter including a first inductor and a first resistance means in series with said first operational amplifier, said first resistance means having first and second values, C. a high-pass filter including a second inductor and a second resistance means in parallel and connected betwEen the output and inverting input of said first amplifiers, said second resistance means having first and second values, D. a feedback circuit including a second operational amplifier for inverting signals from the output of said first amplifier and applying the feedback signals to said low-pass filter, said feedback circuit additionally including means for limiting the feedback signal to a value within the linear operating regions of said operational amplifiers, and E. means responsive to the control signals for simultaneously switching said first and second resistance means between their first and second values simultaneously in response to control signals to thereby vary the oscillator output frequency.
 4. An oscillator as recited in claim 3 wherein each of said first and second resistance means comprises a first resistor and, in parallel therewith, switching means and a second resistor in series, said first and second resistors being paralleled in response to first control signals to thereby increase the natural frequency of said oscillator. 